The invention concerns drying a running web in a multi-cylinder dryer section of a paper making or board making machine. It also concerns the dryer section of the machine.
Moder, high-speed paper making or board making machines have a dryer section, e.g. in accordance with DE-PS 27 30 149, which is equivalent to U.S. Pat. No. 4,183,148. This includes a large number of web dryers, in the form of cylindrical drums of equal diameter, generally either 1.25 m, or 1.5 m or 1.8 m. The dryers are arranged in two rows, one above the other. The web moves over the dryers in series alternating from one row to the other. The dryers are each heated with steam. The first dryers in the series are used to heat up the wet web. They have a lower surface temperature than the succeeding dryers in the series. The temperatures of the dryers usually rise gradually in the direction of web travel. The dryer drive system is combined into groups. The web is guided so as to meander alternately from row to row through the dryer section. In the process, the running web wraps the dryers with approximately the same centri-angle.
Measurements on moist paper webs to be dried on one rotating, heated dryer have shown that the quantity of water evaporated per M.sup.2 (square meter) of paper during the web contact time on the dryer develops as illustrated in FIG. 1 of the drawings hereof in the curves a.sub.1, b.sub.1 and c.sub.1. In FIG. 1, the contact time t is plotted on the abscissa, and the quantity of water evaporated W per m.sup.2 is plotted on the ordinate. The surface temperature of the dryer is constant. The three curves a.sub.1, b.sub.1 and c.sub.1 in FIG. 1 show, from bottom to top, respective lower, middle range and higher paper moistness F at the point where the web begins to contact said one dryer.
It can be seen from the shape of the curves that they run through a turning point WP in which the evaporation rate (that is, the evaporated quantity of water per unit of area and unit of time) of the moisture contained in the web has its maximum value. This insight is new. After the region of the time axis or abscissa corresponding to the maximum evaporation rate, the evaporation rate decreases when the web contacts the dryer for a longer time. It can also be seen that at higher paper moisture (curve a.sub.1), the region of the maximum evaporation rate is reached after a longer contact time t.sub.a than at a lower paper moisture (compare curves b.sub.1 and c.sub.1).
This representation in the curves in FIG. 1 reflects solely its qualitative shape. A quantitative determination of the evaporation of the moisture contained in the web is dependent upon, among other things, the value of the paper moisture, i.e. the percentage of water content, the paper grade and the basis weight, the temperature of the dryer and the contact time of the web on the dryer. The contact time, in turn, depends upon the machine speed, the dryer diameter and the centri-angle of the web wrap on the dryer. The same applies to the drying of board webs.
The curves of the evaporation rate plotted over time can be derived from FIG. 1, that is, the rate curves are the respective derivatives of the quantity curves of FIG. 1. This results in the curves shown in FIG. 2. In these, the contact time t is plotted on the abscissa to the same scale as in FIG. 1, and the evaporation rate V is plotted on the ordinate. Curve a.sub.2 is the higher paper moisture, curve b.sub.2 and curve c.sub.2 are each allocated lower paper moisture in accordance with FIG. 1. From the qualitative shape of the curves in FIG. 2, and provided the dryer temperatures are equal, it can be seen that the region of the maximum evaporation rate (at WP.sub.a) is reached after a longer contact time with the moister paper web on a dryer than with the web of lower moisture content. Again, on the assumption of a constant machine speed and equal centri-angle of the web wrap, it follows that to achieve the maximum evaporation rate V.sub.max over time while the web is running around the corresponding dryer, the moister web must be led around a dryer of larger diameter than the dryer about which a less moist web must be led. To the respective contact time indicated on the abscissa of FIG. 1, for achievement of the maximum value of the evaporation rate V.sub.max is therefore allocated the required corresponding dryer diameter D.
Since, as mentioned above, the dryers on modern, high speed paper or board making machines are of equal diameter, but the web moisture content decreases in the direction of web travel, the dryer section has thus far not been designed accordingly because of the lack of insights about the maximum evaporation rate. In addition, at the dryer diameters and machine speed used, the region of the maximum evaporation rate is not attained at least through major areas of the dryer section.